A First Course on Kinetics and Reaction Engineering Class 35 on - PowerPoint PPT Presentation

A First Course on Kinetics and Reaction Engineering Class 35 on Unit 33

Where We’re Going • Part I - Chemical Reactions • Part II - Chemical Reaction Kinetics • Part III - Chemical Reaction Engineering ‣ A. Ideal Reactors ‣ B. Perfectly Mixed Batch Reactors ‣ C. Continuous Flow Stirred Tank Reactors ‣ D. Plug Flow Reactors ‣ E. Matching Reactors to Reactions • Part IV - Non-Ideal Reactions and Reactors ‣ A. Alternatives to the Ideal Reactor Models - 33. Axial Dispersion Model - 34. 2-D and 3-D Tubular Reactor Models - 35. Zoned Reactor Models - 36. Segregated Flow Model - 37. Overview of Multi-Phase Reactors ‣ B. Coupled Chemical and Physical Kinetics 2

Axial Dispersion Model • The axial dispersion model is a modified form of the plug flow reactor model ‣ it adds a diffusion-like term that accounts for mixing in the axial direction d 2 C i dz 2 + d ( ) = ∑ − D ax ν i , j r j dz u s C i j = all reactions ‣ the quantity D ax is not a diffusion coefficient - it is called the dispersion coefficient - it has one value that is used for all species being modeled - it can be found • from correlations of the axial Peclet number (or its inverse, the dispersion number) as a function of the Reynolds number Pe ax = ul D ax • experimentally, for example using stimulus and response measurements involving a tracer (as is done when measuring the age function • In commercial scale packed bed reactors, including axial dispersion doesn’t affect conversion as long as the reactor length is greater than ca. 50 catalyst particle diameters ‣ The axial dispersion model can still be used to empirically model a reactor ‣ When D ax = 0, it is a PFR; as D ax becomes very large, it approaches a CSTR 3

Boundary Conditions and Numerical Solution • The axial dispersion model is a second order differential equation ‣ There are several ways the boundary conditions could be formulated ‣ Danckwerts boundary conditions are very common dC A ( ) − D ax at z = 0; uC A 0 = u 0 C A , feed dz z = 0 at z = L ; dC A = 0 dz z = L • First Danckwerts boundary condition recognizes that as soon as the feed enters the reactor, it will be diluted by axial mixing ‣ Much like what happens to feed entering a CSTR ‣ It requires that the flow of A upstream of the reactor entrance (where no mixing has occurred) must be equal to the net flow of A (due to convection and mixing) at the inlet to the reactor • Second Danckwerts boundary condition simply requires that the concentration stops changing at the point where the flow leaves the reactor. • The axial dispersion model involves mixed boundary value ordinary differential equations ‣ Numerical solution requires different methods than are used for initial value problems ‣ Discussed in Supplemental Unit S6 4

Questions? 5

Activity 33.1 Suppose liquid phase reaction (1) is second order in the concentration of A. The reaction proceeds isothermally with a rate coefficient equal to 0.05 dm 3 mol -1 min -1 . The liquid density is constant. The feed to the process is taken from a tank containing A at a concentration of 1 M and enters the reactor with a superficial velocity of 6.0 dm min -1 . Plot the concentration of A versus axial position within a tubular reactor for axial dispersion coefficients of 0.1, 1, 10, 100 and 1000 dm 2 min -1 . A → B � (1) Read through the problem statement. Each time you encounter a quantity, write it down and equate it to the appropriate variable. When you have completed doing so, if there are any additional constant quantities that you know will be needed and that can be calculated from the values you found, write the equations needed for doing so. 6

Suppose liquid phase reaction (1) is second order in the concentration of A. The reaction proceeds isothermally with a rate coefficient equal to 0.05 dm 3 mol -1 min -1 . The liquid density is constant. The feed to the process is taken from a tank containing A at a concentration of 1 M and enters the reactor with a superficial velocity of 6.0 dm min -1 . Plot the concentration of A versus axial position within a tubular reactor for axial dispersion coefficients of 0.1, 1, 10, 100 and 1000 dm 2 min -1 . A → B � (1) Read through the problem statement. Each time you encounter a quantity, write it down and equate it to the appropriate variable. When you have completed doing so, if there are any additional constant quantities that you know will be needed and that can be calculated from the values you found, write the equations needed for doing so. k = 0.05 dm 3 mol -1 min -1 C A,feed = 1 mol L -1 u s = 6.0 dm min -1 D ax = 0.1, 1, 10, 100 or 1000 dm 2 min -1 7

Suppose liquid phase reaction (1) is second order in the concentration of A. The reaction proceeds isothermally with a rate coefficient equal to 0.05 dm 3 mol -1 min -1 . The liquid density is constant. The feed to the process is taken from a tank containing A at a concentration of 1 M and enters the reactor with a superficial velocity of 6.0 dm min -1 . Plot the concentration of A versus axial position within a tubular reactor for axial dispersion coefficients of 0.1, 1, 10, 100 and 1000 dm 2 min -1 . A → B � (1) Generate the mole balance design equations needed to model the axially- dispersed tubular reactor by simplification of the general design equations found in Unit 33 or on the AFCoKaRE Exam Handout. 8

Suppose liquid phase reaction (1) is second order in the concentration of A. The reaction proceeds isothermally with a rate coefficient equal to 0.05 dm 3 mol -1 min -1 . The liquid density is constant. The feed to the process is taken from a tank containing A at a concentration of 1 M and enters the reactor with a superficial velocity of 6.0 dm min -1 . Plot the concentration of A versus axial position within a tubular reactor for axial dispersion coefficients of 0.1, 1, 10, 100 and 1000 dm 2 min -1 . A → B � (1) Generate the mole balance design equations needed to model the axially- dispersed tubular reactor by simplification of the general design equations found in Unit 33 or on the AFCoKaRE Exam Handout. d 2 C i d 2 C A dz 2 + d dC A ( ) = ∑ − D ax ν i , j r j − D ax dz 2 + u s dz = − r dz u s C i j = all reactions dC i dC A ( ) − D ax ( ) − D ax at z = 0; u s C i z = 0 = u s C i , feed − u s C A , feed = 0 u s C A 0 dz dz z = 0 z = 0 at z = L ; dC i dC i = 0 = 0 dz dz z = L z = L 9

dC A dC i d 2 C A ( ) − D ax dC A − u s C A , feed = 0 = 0 − D ax dz 2 + u s dz = − r u s C A 0 dz dz z = 0 z = L • Identify the independent and dependent variables, if appropriate, and the unknown quantities to be found by solving the equations 10

dC A dC i d 2 C A ( ) − D ax dC A − u s C A , feed = 0 = 0 − D ax dz 2 + u s dz = − r u s C A 0 dz dz z = 0 z = L • Identify the independent and dependent variables, if appropriate, and the unknown quantities to be found by solving the equations ‣ Independent variable: z ‣ Dependent variable: C A ‣ Solution will give C A ( z ) between z = 0 and z = L • Assuming that the equations will be solved numerically, specify the information that must be provided and show how to calculate any unknown values 11

dC A dC i d 2 C A ( ) − D ax dC A − u s C A , feed = 0 = 0 − D ax dz 2 + u s dz = − r u s C A 0 dz dz z = 0 z = L • Identify the independent and dependent variables, if appropriate, and the unknown quantities to be found by solving the equations ‣ Independent variable: z ‣ Dependent variable: C A ‣ Solution will give C A ( z ) between z = 0 and z = L • Assuming that the equations will be solved numerically, specify the information that must be provided and show how to calculate any unknown values ‣ Must supply code that is given C A and its derivatives as a function of z and uses them to evaluate the ODEs - D ax and u s are known constants - r = k ( C A ) 2 • k is a known constant ‣ Must supply code that evaluates the boundary conditions given values of C A , z and the derivatives at z = 0 and z = L - D ax , C A,feed , u s and L are known constants • Identify what variables will become known upon solving the design equations and show how those variables can be used to answer the questions that were asked in the problem 12

dC A dC i d 2 C A ( ) − D ax dC A − u s C A , feed = 0 = 0 − D ax dz 2 + u s dz = − r u s C A 0 dz dz z = 0 z = L • Identify the independent and dependent variables, if appropriate, and the unknown quantities to be found by solving the equations ‣ Independent variable: z ‣ Dependent variable: C A ‣ Solution will give C A ( z ) between z = 0 and z = L • Assuming that the equations will be solved numerically, specify the information that must be provided and show how to calculate any unknown values ‣ Must supply code that is given C A and z and uses them to evaluate all other quantities in the ODE except derivatives - D ax and u s are known constants - r = k ( C A ) 2 • k is a known constant ‣ Must supply code that evaluates the boundary conditions given values of C A , z and the derivatives at z = 0 and z = L - D ax , C A,feed , and u s are known constants • Identify what variables will become known upon solving the design equations and show how those variables can be used to answer the questions that were asked in the problem ‣ C A ( z ) between z = 0 and z = L will be known, can use it to make the requested plot 13